In the processing of X-ray and photographic films, fixer baths are used to remove silver halides from the film, and such silver compounds remain in solution in the fixer. The desirability of recovering the silver ions present in fixer solutions has long been recognized, both because of the value of the silver and because removal thereof revitalizes the fixer chemicals. The preferred method of recovering silver ions from a solution is by using an electroplating cell because, if operated correctly, an electroplating cell can recover silver of high purity.
Automatic control of the plating current in silver recovery cells has proved to be a significant problem, because photographic fixer solutions also include thiosulfate ions which, if electrolytically decomposed, form sulfides that damage the deposited silver and ruin the fixer solution. If the plating current is accurately controlled, silver can be plated without decomposing thiosulfate ions because silver ions will accept electrons at the cathode of the cell and form metallic silver at a lower threshold voltage (decomposition potential) than that necessary to decompose the thiosulfate. However, when a plating current is driven between the electrodes of a plating cell in the presence of silver ions, the concentration of silver ions drops as the ions are removed by plating, and as the concentration drops, the threshold voltage rises, until it approaches the threshold voltage for decomposing the thiosulfates. It is recognized in the art that the voltage across the electrodes must be maintained at a value below the threshold voltage for decomposing the thiosulfates.
One type of automatic control for a silver recovery system is disclosed in U.S. Pat. No. 3,551,318. In that system, a small reference electrolytic cell having its own anode and cathode is inserted into the solution within the plating cell at a selected location. The reference cell acts as a battery, and provides a voltage which drops when silver ions are introduced into the solution. The amount of voltage drop across the reference cell is used to control the main plating current by selecting the characteristics of a control circuit to cut off the plating current when the voltage across the reference cell rises above a predetermined value. The predetermined cut-off voltage must be arrived at by calculation or experimentation. One disadvantage of such a system is the possibility of contamination of the reference cell electrodes, which can affect the accuracy of the relationship between the reference cell voltage and the concentration of silver ions in the solution. Other disadvantages are the measurement of the concentration of silver ions at an arbitrary point within the cell rather than at the critical location adjacent to the cathodic plating surface, and the reliance on a calculated or experimentally determined fixed reference voltage at which the main plating current should be cut off.
Another type of silver recovery control is disclosed in U.S. Pat. No. 3,875,032. That system uses an auxillary anode and cathode to monitor the amount of current that will flow through a solution between the auxillary electrodes at a fixed reference voltage that is selected to be close to the threshold voltage for decomposing thiosulfates when no silver is present in the solution. The plating current is then controlled by a control circuit as a function of the measured current flowing between the auxillary electrodes. That is, when the measured current indicates that the threshold voltage for plating silver is becoming dangerously high, the plating current is cut off.
The system of U.S. Pat. No. 3,875,032 has disadvantages that are similar to the system first described. The concentration of silver ions is measured at a point remote from the actual plating surface and the measured concentration may vary from the concentration at the plating surface. Again, the system relies on calculated curves for determining the plating current cut-off point.
Laboratory electrodeposition techniques using controlled potential electrodes have been used to separate metal ions having relatively close decomposition potentials. In this method, any difference in the potential of a calomel reference electrode and the cathode is measured and used to increase or decrease the voltage driving the electrolysis current to maintain the cathode at the correct potential to carry out the desired deposition without undesirable decomposition.
No previous application of the controlled potential electrode technique to silver recovery from photographic chemical baths is known. We have found that the use of such technique without the modifications of the present invention fails to provide a desired efficiency of silver collection at concentrations of silver ions in solution above about 0.1 grams per liter, because the plating current is not permitted to rise to high enough values. It is believed that the continuous flow of plating current interferes with the ability of the reference electrode to accurately reflect higher silver ion concentrations that can support a relatively high plating current.